Dark matter fraction of low-mass cluster members probed by galaxy-scale strong lensing

We present a strong lensing system composed of four multiple images ofa source at z = 2.387 created by two lens galaxies G1 and G2belonging to the galaxy cluster MACS J1115.9+0129 at z = 0.353. We useobservations taken as part of the Cluster Lensing and Supernova surveywith Hubble and its spectroscopic follow-up programme at the Very LargeTelescope to estimate the total mass distributions of the two galaxiesand the cluster through strong gravitational lensing models. We findthat the total projected mass values within the half-light radiiR_e of the two lens galaxies areM_TG1(<R_e G1) = (3.6 ± 0.4) ×10^10 M_⊙ and M_TG2(<R_eG2) = (4.2 ± 1.6) × 10^10M_⊙. The effective velocity dispersion values of G1 and G2are (122 ± 7) km s^-1 and (137 ± 27) kms^-1 respectively. We remark that these values are relativelylow when compared to those of ≈200-300 km s^-1 typical oflens galaxies found in the field by previous surveys. By fitting thespectral energy distributions of G1 and G2 we measure projectedluminous over total mass fractions within R_e of 0.11 ±0.03 for G1 and 0.73 ± 0.32 for G2. The fact that the lessmassive galaxy G1 is dark matter-dominated in its inner regions raisesthe question of whether the dark matter fraction in the core ofearly-type galaxies depends on their mass. Further investigating stronglensing systems will help us understand the influence that dark matterhas on the structure and evolution of the inner regions of galaxies.